FEATURE: Allow themes to specify modifiers in their about.json file (#9097)

There are three modifiers:
- serialize_topic_excerpts (boolean)
- csp_extensions (array of strings)
- svg_icons (array of strings)

When multiple themes are active, the values will be combined. The combination method varies based on the setting. CSP/SVG arrays will be combined. serialize_topic_excerpts will use `Enumerable#any`.

app/models/theme.rb

@@ -17,6 +17,7 @@ class Theme < ActiveRecord::Base
   has_many :parent_themes, -> { order(:name) }, through: :parent_theme_relation, source: :parent_theme
   has_many :color_schemes
   belongs_to :remote_theme, dependent: :destroy
+  has_one :theme_modifier_set, dependent: :destroy
 
   has_one :settings_field, -> { where(target_id: Theme.targets[:settings], name: "yaml") }, class_name: 'ThemeField'
   has_one :javascript_cache, dependent: :destroy
@@ -34,6 +35,10 @@ class Theme < ActiveRecord::Base
     changed_schemes << scheme if scheme
   end
 
+  def theme_modifier_set
+    super || build_theme_modifier_set
+  end
+
   after_save do
     changed_colors.each(&:save!)
     changed_schemes.each(&:save!)
@@ -44,6 +49,8 @@ class Theme < ActiveRecord::Base
     changed_fields.each(&:save!)
     changed_fields.clear
 
+    theme_modifier_set.save!
+
     if saved_change_to_name?
       theme_fields.select(&:basic_html_field?).each(&:invalidate_baked!)
     end
@@ -112,8 +119,8 @@ class Theme < ActiveRecord::Base
   end
 
   def self.get_set_cache(key, &blk)
-    if val = @cache[key]
-      return val
+    if @cache.hash.key? key.to_s
+      return @cache[key]
     end
     @cache[key] = blk.call
   end
@@ -247,6 +254,15 @@ class Theme < ActiveRecord::Base
     (@cache[cache_key] = val || "").html_safe
   end
 
+  def self.lookup_modifier(theme_ids, modifier_name)
+    theme_ids = [theme_ids] unless Array === theme_ids
+
+    theme_ids = transform_ids(theme_ids)
+    get_set_cache("#{theme_ids.join(",")}:modifier:#{modifier_name}:#{ThemeField::COMPILER_VERSION}") do
+      ThemeModifierSet.resolve_modifier_for_themes(theme_ids, modifier_name)
+    end
+  end
+
   def self.remove_from_cache!
     clear_cache!
   end
@@ -521,6 +537,13 @@ class Theme < ActiveRecord::Base
         end
       end
 
+      meta[:modifiers] = {}.tap do |hash|
+        ThemeModifierSet::MODIFIERS.keys.each do |modifier|
+          value = self.theme_modifier_set.public_send(modifier)
+          hash[modifier] = value if !value.nil?
+        end
+      end
+
       meta[:learn_more] = "https://meta.discourse.org/t/beginners-guide-to-using-discourse-themes/91966"
 
     end